1. Field of the Invention
This invention relates to an apparatus and method for supplying, mixing and dispensing a two-component bons cement.
2. Background Information
In many orthopedic surgical procedures, it is necessary to fix a prosthesis, such as an artificial joint, to living bone within a patient. Such procedures are used to treat osteoarthritis, rheumatoid arthritis, traumatic arthritis, avascular necrosis secondary to sickle cell anemia or collagen disease, severe joint destruction secondary to trauma or other conditions, and the revision of previous arthroplasty procedures. For such procedures, the use of a bone cement, in the form of an acrylic material comprising a reaction product of a methylmethacrylate monomer and a polymethyl-methacrylate-methylmethacrylate-styrene copolymer, is indicated. The use of such a bons cement is also indicated for the fixataon of pathological fractures where loss of bone substance or recalcitrance of the fracture renders more conventional procedures ineffective. A bons cement of this type allows the seating and securing of a prosthesis to bone, and, when polymerization is complete, provides a buffer for the even distribution of weight and other stresses between the prosthesis and bone.
A bone cement of this type is available from Howmedica, a Division of Pfizer Hospital Products Group, Inc., Rutherford, N.J., under the registered trademark SURGICAL SIMPLEX.RTM. P, a radiopaque bone cement. This material is packaged as two separate, sterile components, which must be thoroughly mixed immediately before the cement is applied in surgery.
The first of these components is a colorless, flammable liquid, having a sweet, slightly acrid odor, supplied in an ampule containing a 20-ml full dose or a 10-ml half dose. This liquid component is composed of methyl methacrylate monomer, 97.4 percent by volume, N, N-dimethyl-p-toluidine, 2.6 percent by volume, and hydroquinone, 75.+-.15 parts per million. The hydroquinone is added to prevent premature polymerization, which may otherwise occur under certain conditions, such as exposure to light or elevated temperatures. The N, N-dimethyl-p-toluidine is added to promote the cold curing of the mixed compound. This liquid component has been sterilized by membrane filtration.
The second of these components is a finely divided white powder, supplied in a packet containing a 40-g full dose or a 20-g half dose. This powder component is composed of polymethyl methacrylate, 15 percent by weight, methyl methacrylatestyrene copolymer, 75 percent by weight, and barium sulfate, U.S.P, 10 percent by weight. The barium sulfate, U.S.P. is added to provide radiopacity. This powder component is sterilized by gamma irradiation.
When these liquid and powder components are mixed, an exothermic polymeric reaction forms a soft, pliable, dough-like mass. Within a few minutes, as this reaction progresses, a hard, cement-like complex is formed. These components may be manually mixed in a stainless steel or other inert container, into which the entire content of the powder component package is poured. The entire liquid content of the ampule is added to this powder, and the mixture is stirred with a sterile stainless steel spatula, or other suitable inert device, until the powder is completely saturated with the liquid. The mixture is then administered to the bone under pressure through a suitable sterile injection device, after stirring for 1 to 11/2 minutes in an operating room temperature of 75.degree. F.
Alternately, the mixture may be manually administered, after stirring is continued until a dough-like mass is formed. After a mixing and kneading process at least 4 minutes in duration, the fact that the mass does not stick to the rubber gloves of the operator provides an indication that the mass is ready for manipulation. The mixed and kneaded cement is applied to the bone manually or through the use of a suitable sterile pressurizing device, with the correct working consistency of the cement for application to bone being best determined by the experience of the surgeon.
Because the mixed cement sets so quickly, the two components are almost always mixed in the sterile environment of the operating room by a surgical assistant. A number of precautions must be followed. For example, the liquid component is a powerful lipid solvent, which has caused contact dermatitis in susceptible individuals. It is recommended that an individual working with these materials should wear two pairs of surgical gloves, while strictly adhering to established procedures, so that the compound dos not come into direct contact with sensitive tissues, and so that is not absorbed in his body. During the mixing of these materials, and during their chemical reaction, noxious and toxic vapors may be produced, which preclude, for example, the wearing of soft contact lenses in the operating room where the cement is being prepared. Although there is no present report indicating that the vapor is harmful, it is unpleasant, and it disturbs the breathing of many individuals. It is therefore particularly desirable to minimize the contact of operating room personnel with the vapor and to minimize the release of the vapor into the atmosphere.
Care must also be taken in the mixing of the liquid and powder components to ensure that the entire contents of the ampule and pouch are utilized. The mixing of these components should be thorough and vigorous. Data from in vitro studies have shown that monomer loss is related primarily to the frequency of stirring and secondarily to the duration of stirring. Adverse reactions affecting the cardiovascular system have been attributed to a leakage of unpolymerized liquid monomer into the circulatory system. On the other hand, caution should be taken to avoid kneading the cement too long, in order to avoid progression of the polymerization process to the point at which the cement is not adequately soft and pliable to obtain good filling of the bone cavities and fitting to the prosthesis.
3. Description of the Prior Art
In the U.S. Patent art, a number of examples describe apparatus for mixing the components of bone cement in an evacuated chamber. In a first example of such apparatus, U.S. Pat. No. 4,185,072 to Puderbaugh et al. discloses a manually-operated combination mixing-reaction apparatus including a housing with an intermediate cavity shaped generally as a hemispherical bowl. An annular pattern of channels extending around the top of this intermediate cavity is connected with a lower cavity, in which a vacuum is drawn. A cover placed atop this structure provides a central bearing for a shaft extending downward to mixing vanes and upward to a pivoting handle. One mixing vane includes an arcuate portion rotated close to the hemispherical wall of the intermediate cavity, while the other mixing vane includes an arcuate portion extending midway between the wall and the vertical axis of rotation of the vanes.
Another example of such apparatus is found in U.S. Pat. No. 5,348,391, to Murray, which describes a rotary mixer having a pair of mixing arms. The arms follow continuous paths within a cylindrical container in which the cement is mixed, with each such path having a plurality of loops at the chamber and curved portions extending across the chamber between loops. This type of movement is accomplished by driving the arms in a planetary motion, with rotation of a planetary shaft to which the arms are attached occurring within a plate rotated about a central drive shaft turned with a crank extending above the chamber. The planetary rotation is caused by the meshing engagement of several gears. The arms move through the cement, breaking bubbles and thoroughly mixing the cement components. An opening at the top of the chamber is connected to an outer chamber in which a vacuum is drawn.
While the devices of U.S. Pat. Nos. 4,185,072 and 5,348,391 each contain vanes or arms operating in a paddle fashion to mix the cement components, the contact between these movable mixing devices and the wall of the container in which the cement is mixed is limited, if such contact occurs at all. It is particularly difficult, using mass production methods, to form a container of this sort with accurate and repeatable internal dimensions. Since the vanes and arms of these patented prior art devices are relatively rigid, clearance must be provided between these vanes and arms and the walls, in order to prevent the jamming of the mixing mechanism due to mechanical interferences. Thus, what is needed is a mixing device presenting a flexible surface to slide and scrape along the surface of the mixing chamber.
Furthermore, to facilitate the maintenance of sterile conditions in the preparation of bone cement, it is particularly desirable to provide one of the components used to form the bone cement within a disposable mixing chamber. The mechanical complexity of the device of U.S. Pat. No. 5,348,391 tends to make the device non-disposable, and the meshing gears may not be compatible with the storage and shipment of a powdered component within the mixing chamber.
U.S. Pat. No. 4,671,263 to Draenert describes a pistol-shaped device for applying previously-mixed bone cement under pressure. The cement, which is mixed, for example, manually in a bowl external to the device, is introduced to a generally-cylindrical, sealable container forming a part of the device. This container is removably attached to a housing including a handle. A tapered distal end of the container is sealed by a removable closure cap. A pneumatically-operated sliding ejector extending initially within the housing engages a lamellar structure acting as a piston slidably mounted within the container, originally at a proximal end of the container. The cement mixture is prepressurized for about five minutes by means of a force exerted on the lamellar structure by the sliding ejector. During this process, gasses entrapped within the mixture escape past the lamallae of the lamellar structure along the inner surface of the container, while the lamallae prevent the escape of the cement mixture. The force exerted by the sliding ejector is then relieved, and the closure cap is removed, and the cement mixture is injected from the device by the repeated opening of a valve causing the pneumatic operation of the sliding ejector. On one version of the device, the container is additionally rotated about its axis.
While the device of U.S. Pat. No. 4,671,263 particularly addresses means for conditioning the previously-mixed bone cement before it is administered into the patient, and means for dispensing the mixture into the patient, the mixing of components of the cement remains a separate operation, to be carried out in separate apparatus. When the mixture is transferred from the mixing apparatus to the device described in this patent, it is exposed to the air, and hence to potential contamination and to polymerization problems which may occur with undue exposure to air. Furthermore, during this transfer of material, the air within the operating room is exposed to the mixture, and hence to the release of noxious and toxic gasses generated during the polymerization process. What is needed is apparatus providing an enclosure in which both mixing and application can occur.
While various devices from the prior art, including those including those of the patents described above, use a vacuum to prevent the entry into the ambient air of noxious gasses produced during the mixing process, none of these devices include means for indicating that an effective vacuum is being produced. The vacuum may be rendered ineffective by the failure of a pump or be the physical clogging of the apparatus at a number of points. This, what is needed is a mechanism providing a visual indication that the required vacuum is being produced. U.S. Pat. No. 4,277,184 to Solomon describes an orthopedic implement providing a disposable, closed system in which bore cement is mixed and from which the mixed cement is dispensed. The closed system includes a chamber, a member reciprocable within the chamber, a mixing member operatively and axially receivable within the reciprocable member, and means for rotating the mixing member within the chamber. This means for rotating may be a pneumatic or electric drill. When the implement is to be used, the two components to be mixed are poured into the chamber, and the reciprocable member is inserted into the chamber with the mixing member extending through the reciprocable member. The mixing member is connected to the drill, being rotated thereby so that a pair of triangular vanes extending from the mixing member are rotated within the chamber. After mixing is complete, the mixing member is pulled outward so that its vanes are withdrawn into slots in the reciprocable member, a plug closing a lower opening to the chamber is removed, and the reciprocable member is pushed into the chamber, pushing the mixed cement outward therefrom, through the lower opening, and through a dispensing tube attached thereto.
While the device of U.S. Pat. No. 4,277,184 provides a closed chamber for mixing and dispensing bone cement, the containers in which the two components of the cement are supplied must be opened externally to the disposable implement and poured into the chamber. This part of the process exposes these components to possible contamination. What is needed is a means for providing these components within the implement and for opening packages within the implement as necessary before the mixing process is begun. Furthermore, since the triangular paddles sweep through only a small portion of the space within the chamber as the mixing member is rotated, and since there is no member scraping along the walls of the chamber during the mixing process, it is believed that mixing may not be optimally vigorous and through.
U.S. Pat. No. 4,463,875 to Tepic describes bone cement mixing apparatus in which the two components of the cement are vacuum-packaged in elongated flexible, fluid-tight compartments, which are abutted in a side-to-side arrangement to one another, and which are sealed together. These compartments are acted upon from each side by a dual piston arrangement including a hemispherically-shaped outer piston and a smaller inner piston. The inner piston may travel with the outer piston, or it may be extended into the adjacent compartment independently from the outer piston. The first piston movement collapses one of the compartments, rupturing the walls between the compartments, so that the components therein are mixed. Then, the two compartments are alternately collapsed and controlledly extended until the components therein form a homogenous cementitous mixture. Next, a nozzle is attached to one of the compartments in lieu of the other compartment, and the one compartment is collapsed to expel the mixture to the deposition site.
While the method of U.S. Pat. No. 4,463,875 provides the advantage of carrying out the mixing process before either of the components or the mixture is opened to the atmosphere, the apparatus required to carry out this method is complicated by the fact that two piston motions are required from each end of the compartments, which must therefor be placed centrally within the apparatus. Furthermore, no means is provided for establishing a vacuum before or after the mixing process, so noxious gasses resulting from the mixing process are released into the ambient air when the compartments are separated to install the nozzle. Also, there is no way to clear portions of the cement mixture remaining on the inner walls of the compartments following the mixing process. Also, providing the components together in a single, although divisible, package is complicated by the fact that they cannot be sterilized together, since the sterilization processes for the liquid and powder components are different and incompatible. That is, the liquid component cannot be exposed to the process presently used to sterilize the powder component, and the powder component cannot be exposed to the process presently used to sterilize the liquid component. Thus, what is needed is an apparatus holding both of the components of the cement, to which mixing motions are provided from a single end. Also, what is needed is a method for applying a vacuum to a chamber in which the components are mixed during the mixing process, and a means for scraping the inner wall of such a chamber to remove the mixture following the mixing process.
A method for advancing a shaft in a predetermined direction through a number of incremental manual motions of a crank is described in U.S. Pat. No. 5,579,694 to Holung et al, in an application involving the attachment of a shutter to windows of varying width. This method, or a variation thereof, may be applied in a device using the movement of a piston engaged by an advancing shaft to dispense bone cement.